Silicon photonics has gained increased attention in recent years because of its potential to significantly reduce the cost of optical devices used in traditional applications and its potential to enable new applications. Silicon Photonic Integrated Circuits (PICs) can be fabricated using standard, wafer scale CMOS technology, allowing a drastic reduction of the cost of the integrated circuits through economies of scale. Moreover the high-end fabrication tools, also used for fabricating the most advanced electronic chips and capable of handling 200 mm or 300 mm silicon wafers, allow achieving high-yield fabrication and can bring volume economies to optics. Silicon is an ideal material platform for near-infrared and mid-infrared photonic integrated circuits. With a band gap of 1.12 eV, silicon is transparent for telecom wavelengths such as 1.3 μm and the 1.55 μm band. Moreover, using the silicon platform, integration of photonic devices with microelectronic circuits is enabled. Silicon photonics can enable a chip-scale platform for monolithic integration of photonics and micro-electronics for applications of optical interconnects in which high data bandwidths are required in a small footprint or applications such as chemical and biological sensors, or signal processing, where lower speed digital or analog functions may be required.
Many basic building blocks have been demonstrated in silicon photonics. High speed, 40 Gb/s, modulators, and photo-detectors have been demonstrated as well as passive devices such as multiplexers and demultiplexers, leading to high bandwidth silicon photonic chips. Electrically pumped lasers and optical amplifiers of a group IV element are not yet demonstrated. Various options have been suggested for such devices, one example being fiber coupling an external packaged laser to the silicon chip. This has an advantage in terms of thermal isolation but is not an integrated solution and so has a larger footprint. Furthermore, there is an important fiber coupling cost.
Individual laser dies also can be flip chip mounted to the optical wafer or might be transferred to the silicon platform using an epitaxial lift-off process. Tight mechanical alignment requirements make this option less attractive especially for wavelength division multiplexing (WDM) systems wherein the number of lasers required per chip for most applications has increased over the last years.
Alternatively, two different bonding approaches for achieving hybrid integration of III-V lasers with silicon-on-insulator (SOI) waveguides without need for stringent alignment in the assembly process have been published: molecular direct bonding and adhesive bonding. The advantage of such bonding approaches is that the alignment between III-V structures and silicon structures is achieved collectively through lithography. Furthermore, many lasers may be fabricated by bonding a single III-V die or wafer onto the silicon chip giving the possibility of fabricating arrays of multi-wavelength lasers integrated with other silicon photonic components suitable for WDM links. In the molecular direct bonding approach a strong bond between the different wafers/dies is realised by interfacial bonds. Adhesive bonding technology uses a glue, e.g. polymers or metals to realise wafer bonding.
At present some laser concepts have been enabled using heterogeneous integration. A first example of hybrid devices that have been reported are single and array micro disk lasers and FP lasers. The devices are all characterized by the fact that laser characteristics, e.g. laser wavelength, is defined by the III-V disk or ring or ridge. For these lasers, the in plane waveguide confinement and cavity confinement are performed in III-V materials.
A second example of hybrid devices provides in plane waveguide confinement and cavity confinement in silicon. In the second example, the device comprises a III-V epiwafer or die with a multiple quantum well region bonded prior to patterning the III-V into a photonic device, by means of a direct or adhesive bonding technology to the silicon platform. Patterns are defined in the silicon platform prior to the bonding process. These patterns might be essential parts of the laser cavity e.g. gratings, facets, tapers. Often, such a device is characterized by the fact that the optical mode is guided in the silicon waveguide. The optical mode of the silicon waveguide at least partially overlaps the III-V layer stack, realizing a confinement factor of about 5% with the multiple quantum well region (with typically 4 to 6 quantum wells). The optical mode obtains gain from the active region of the electrically pumped gain medium material while being guided by the optical waveguide in the passive semiconductor material or silicon waveguide. The silicon waveguide mode is evanescently amplified by the III-V epilayer, and such devices are as such called evanescent lasers. The cavity is formed by structures defined in the silicon platform, e.g. gratings, tapers, facets. The light is coupled out of the laser through one of these facets in the silicon passives. Different hybrid silicon-based evanescent devices are known e.g. Distributed Feedback Laser (DFB), Distributed Bragg Reflector laser (DBR), Mode-locked multi-wavelength laser, etc. Besides evanescent lasers, hybrid evanescent amplifiers, and/or detectors are known, all based on an evanescent overlap of the silicon waveguide mode with the III-V material. Hybrid silicon-based evanescent devices intrinsically have a low multiple quantum well confinement factor (order of 5%) by the fact that the optical mode is guided by the optical waveguide in the passive semiconductor material, e.g. silicon waveguide, and evanescently overlaps the active semiconductor material, e.g. III-V QW region. The confinement factor in the quantum wells is critical for determining how much gain/absorption is achievable. Once the gain from III-V active region is equal to the cavity and mirror loss, lasing is achieved. The low multiple quantum well confinement factor results in a moderate modal gain. This design choice targets minimal coupling losses from the laser section towards the adjoining optical network or circuit. The silicon confinement factor is an important parameter determining coupling efficiency with the adjoining optical network or circuit. To realize efficient coupling, a good confinement in the passive silicon waveguide is necessary (order 50-70%). Hybrid silicon evanescent lasers need to choose a compromise between silicon waveguide confinement and quantum well confinement.
There is still a need for efficient hybrid silicon photonics optical devices and/or hybrid silicon photonics optical devices operating at low overall power consumption.